Ben More Assynt

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At 998m, Ben More Assynt is the highest peak in the district. Consequently an ascent of it and its lofty neighbour, Conival (987m), is much prized by hillwalkers. And rightly so for the walk gives superb views west to the coast, south to the hills inland from Ullapool and north towards Foinaven. But the ascent, from the normal starting point at Inchnadamph, takes in some great geology too.
It crosses from the foreland, through a suite of minor thrust sheets that greatly thicken up the Durness carbonates, then up through the Glencoul sheet an on across the Ben More Thrust. However, the highlights are all up high, within the Ben More Thrust sheet.

The Ben More Thrust Sheet is one of the major thrust sheets in Assynt. It carries a slice of Lewisian basement together with its overlying sedimentary cover. This includes a thin tract of Torridonian together with the overlying Cambrian quartzites. The structure of these units is particularly interesting. The area around Conival contains the site where the Lewisian units taper out against the thrust. To the west Cambrian quartzites lie directly above the Ben More Thrust. This is an example of a "hanging-wall ramp" and the related folding is spectacularly exposed on the south face of Na Tuadhan (860m, a subsidiary summit north of Conival). Indeed a photograph of this dramatic structure formed the frontispiece to the classic 1907 Geological Memoir.

Torridonian rocks along the Ben More Thrust are exposed in upper Coire a'Mhadaidh. Pebbles within conglomeratic beds are strongly flattened, demonstrating that significant strains are associated with folding. The Torridonian strata are only found near the thrust. Further east Cambrian quartzites lie directly on Lewisian gneisses. This type of "double unconformity" relationship is also found in the foreland (e.g. on the south side of Loch Glencoul, leading up to Quinag).

To appreciate the geometry of the Ben More Thrust it is necessary to climb high on the hill. Although it maps round onto the SW face of Conival, which is itself very prominent from the shore of Loch Assynt, the thrust is hard to spot because it places quartzites on quartzites. Nevertheless there is worthwhile geology to be found on the approach walk up the Traligill valley. Most notable is the thrust plane that repeats part of the Durness carbonate sequence that crops out in the floor of the valley [e.g. NC 270209]. The thrust surface contains both a sink and resurgence in the stream. This structure has been much photographed and has illustrated some classic text-books. Consequently, together with all the high ground around Ben More and Conival, it is protected as a Site of Special Scientific Interest. Consequently there should be no collecting.

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